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Full text of "Text Book Of Mechanical Engineering"

1182                          Appendix  VI.

second is obtained by the use of a vacuum of 28", shewing the
importance of condensation. A de Laval nozzle would have a
length represented by o t if condensing, but of o n if non-condensing.
(2) Dividing the velocity curve into equal vertical drops, we shew
a number of shadings that indicate the wheels of a multiple-stage
turbine, when each absorbs an equal change of velocity. It
will be seen that 40,000 ft. pds. require ^ wheels at first, 2 after-
wards, and finally one, which means that the later wheels are more
efficient, and that the length o q should be replaced by an intro-
ductory nozzle.

The design of a steam turbine is too extensive a subject to
enter on here, but Fig. 1037 may be referred to as shewing the
way in which the velocities change as the fluid passes through the
wheels. Taken very simply in the first case as an impulse turbine
with non-expansible fluid, the steam enters through the first set
of guide blades, and passes into the first wheel with an actual
velocity a which becomes a relative velocity of r if wheel
velocity is w. Entering the second guide, where a% is the actual
velocity, equal to r^ and placed at guide angle, this is changed to
the relative velocity r2. Again r% is put as a3 to enter the second
wheel, and this leaves the wheel as r3 but at angle of a4 to enter
the next set of guides : and so on. The limit is reached when
a~w. To be correct, however, this construction must be used
in. conjunction with the velocities as shewn in Fig. 1036, which
increase as the fluid expands, and more wheels would therefore
be required than are suggested. In a reaction turbine #2 is
greater than r and as than rz in a relation depending on the
extent to which the passages are choked. The energy taken by
the wheels will be proportional to af - a.

Fluid friction is very great at the high speeds involved, and
this will modify the calculations considerably.

Pp. fog, 9/5, and 999. Petrol Motor Cars. The
general form of the chasm of a modern car has settled down, and
extraneous types are gradually disappearing. The latter is un-
fortunate where such types are clever and well-working, but the
demands of economical manufacture make some sort of standard-
isation desirable. The Live-axle Car has developed simulta-
neously with the Chain-driven Car, neither being now belonging to